The present invention relates to reflective optical elements for a working wavelength in the soft X-ray and extreme ultraviolet wavelength ranges, in particular for use in an EUV lithography apparatus comprising a first multilayer system of at least two alternating materials having different real parts of the refractive index at the working wavelength on a substrate, which exerts a layer stress on the substrate, and comprising a second multilayer system of at least two materials, periodically alternating, as the case may be, on a substrate, which exerts an opposed layer stress and is arranged between the first multilayer system and the substrate. Moreover, the present invention relates to a projection system, in particular for an EUV lithography apparatus, comprising at least one of the above-mentioned reflective optical elements, an illumination system, in particular for an EUV apparatus, comprising at least one of the above-mentioned reflective optical elements, a beam-shaping system, in particular for an EUV lithography apparatus, comprising at least one of the above-mentioned reflective optical elements, and an EUV lithography apparatus, comprising at least one of the above-mentioned reflective optical elements.
In EUV lithography apparatuses, reflective optical elements for the extreme ultraviolet (EUV), or soft X-ray wavelength range (e.g. wavelengths between about 5 nm and 20 nm), such as photomasks or mirrors, are used for the lithographic processing of semiconductor components. Since EUV lithography apparatuses usually have a plurality of reflective optical elements, they must have the highest possible reflectivity in order to ensure sufficient overall reflectivity. Since a plurality of reflective optical elements are usually arranged in series in an EUV lithography apparatus, the slightest deterioration in reflectivity of each individual reflective optical element has a substantial effect on the overall reflectivity within the EUV lithography apparatus.
Reflective optical elements for the EUV and soft X-ray wavelength range, usually comprise highly reflective coatings in the form of multilayer systems. These are alternately applied layers of a material having a higher real part of the refractive index at the working wavelength (also referred to as spacer) and a material having a lower real part of the refractive index at the working wavelength (also referred to as absorber), wherein an absorber-spacer pair forms a stack, or a period. This essentially simulates a crystal, wherein its lattice planes correspond to the absorber layers, on which Bragg reflection occurs. The thicknesses of the individual layers as well as the repetitive stacks can be constant across the entire multilayer system, or they can vary depending on which reflection profile is to be achieved.
As early as during the coating process, a stress can build up within a multilayer system, which has an effect on the underlying substrate and deforms it to an extent that the optical imaging on the corresponding reflective optical element is substantially interfered with. The type of stress depends, amongst other things, on the materials used as the spacer and the absorber and the thickness ratios present within a stack, or a period. Usually, between the highly reflective multilayer system and the substrate, an interposed coating is provided, which has a layer stress opposed to the one exerted by the reflective multilayer system and which can also be formed as a multilayer system of alternating layers of different materials.